1. Field of the Invention
This invention relates to oil wells and more particularly to a system for control of the operation of reciprocating downhole oil well pumps.
2. Description of the Prior Art
Although many oil wells will initially flow continuously because of the high pressure on the oil in the underground reservoirs penetrated by the well, there is a large number of wells that will produce only if oil is pumped from the borehole of the well. Those wells that are initially free flowing usually must be pumped after the reservoir has become partially depleted. The most common type of pump is a reciprocating pump that is installed at a depth in the well such that oil will flow from the surrounding reservoir into the borehole of the well and submerge the lower end of the pump. A plunger in the pump is connected through sucker rods extending upwardly through the well and a wellhead at the upper end of the well to a pumping unit driven by a suitable prime mover such as an electric or internal combustion motor.
If the maximum production from the well is to be obtained, the rate at which the pump will deliver oil to the surface is higher than the rate at which oil will flow into the well from the surrounding reservoir. To prevent operation of the pumping unit when there is insufficient liquid in the well for efficient pumping of well fluids, the pumping unit is periodically shut down to permit the accumulation of well fluids, particularly oil in the well. Because many wells are in remote locations, it is desirable to provide a system that will automatically control the operation of the pump to maintain a pumping efficiency above a preselected minimum.
In U.S. Pat. No. 2,550,093 of Smith, a system is disclosed in which the well fluids are pumped against a spring-loaded check valve. Operation of the pumping unit is controlled by a timer that cuts off the power supply to the pumping unit after pumping has proceeded continuously for a preselected period. Each pressure pulse from a stroke of the pump that opens the check valve causes resetting of the timer to the starting point of the preselected period.
In apparatus described in U.S. Pat. No. 3,274,940 of Cottrell, the pumping unit is controlled by a float-operated switch in a chamber in the discharge line from the well. If the downhole pump does not deliver liquids from the well at a rate high enough to cause the float chamber to fill to a level to actuate the float-operated switch, the pumping unit is shut down. A timer restarts the pump after it has been shut down for a preselected period. Other flow control systems are described in U.S. Pat. No. 2,690,713 of Urmann et al. in which the level of liquid in the well initiates a signal to control the pump operation; U.S. Pat. No. 3,075,466 of Agnew et al. in which variations in the current drawn by the electric motor for operating the pumping unit, etc. are used to control the motor of the pumping unit; and U.S. Pat. No. 3,854,846 of Douglas in which pressure pulses are used to reset a timer in a control system for the pumping unit. U.S. Pat. No. 3,854,846 discloses a specific electrical circuit for permanently shutting down the system until it is manually restarted if several restarts fail to cause flow high enough to actuate a reset signal.